McMahon told reporters that the shooter apparently fatally shot a man and woman -- both store employees -- before shooting himself in a store on the second floor of The Mall in Columbia. The motive was unclear.

Police identified the dead employees as 21-year-old Brianna Benlolo and 25-year-old Tyler Johnson. Both worked at Zumiez, a shop that caters to skaters, where the shootings took place.

Benlolo was an assistant manager at the store and had worked there since November 2012, according to her Facebook page.

Johnson had worked at the store for about three months, according to his Facebook page.

The body of the suspected shooter was found near a shotgun on the floor of Zumiez, police said, along with unspent ammunition.

Late Saturday, police revealed that "two crude devices that appeared to be an attempt at making explosives using fireworks" were found inside the shooter's bag inside the store. "Both were disabled," police added.

Thousands in mall at time of shooting

The first 911 call about the shooting came at about 11:15 a.m. and officers were in the mall within two minutes, police said.

The shooting "seems to have been very contained to that store and the area just outside," McMahon told reporters.

Police entered the mall within minutes of the first 911 calls and found three people dead. The scene was secure shortly before 1 p.m., police said.

Investigators said there were thousands of people in the mall at the time, with many hiding in fear behind store counters, in restrooms or fitting rooms for hours after the shooting stopped.

"Think about this, on a Saturday afternoon at the mall, how many people may be in there," McMahon said. "Something like this happens and people run in many directions, and they also do what we train them to do -- to shelter in place."

A federal official briefed on the shooting told CNN that preliminary information indicates the shooting may have been related to a domestic dispute. Police, however, said as late as Saturday night that they don't have a motive, nor have they indicated whether or not the shooter and victims even knew each other.

"We do not know yet what caused the incident," said McMahon.

Five people were transported to Howard County General Hospital, a hospital spokeswoman said in a statement. All were treated and later released.

Four of them suffered injuries related to the chaotic scene after the shots rang out, including one with a seizure and at least one more with a sprained ankle, according to McMahon.

One of the victims suffered a gunshot wound to the foot. Police revealed Saturday night that this woman wasn't in Zumiez; rather she was in the first floor when she was struck.

Worker at mall: 'It was just crazy'

The gunfire sent shoppers and workers running for cover, witnesses told CNN.

"It's a mall shooting," one mall worker, identified only as K.T., told CNN. "No one knows what's going on. In today's world, you hear gunshots and you run."

Photos: Shooting at Maryland mall 14 photos

Photos: Shooting at Maryland mall14 photos

Shooting at Columbia Mall in Maryland – Police evacuate employees and patrons from the Columbia Town Center Mall after a shooting resulting in fatalities there on January 25 in Columbia, Maryland.

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Photos: Shooting at Maryland mall14 photos

Photos: Shooting at Columbia Mall in Maryland – The Maryland State Police medivac flys over the mall.

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Photos: Shooting at Maryland mall14 photos

Photos: Shooting at Columbia Mall in Maryland – Police escort people from the mall as they assist in evacuations.

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Photos: Shooting at Columbia Mall in Maryland – People leave the mall as it is evacuated after the shooting.

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Photos: Shooting at Columbia Mall in Maryland – The area outside the mall is seen after the shooting.

Deadly shooting at suburban mall

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Mall shooting happened in one store

The staccato of gunfire was followed by the cries and screams of children and adults running or ducking for cover, the employee said.

"A lot of kids were crying, and mothers were holding onto them," K.T. said. "I wasn't worried about me. I was just making sure everybody was OK."

Once the shooting stopped, SWAT team members moved from store to store.

"It was just crazy," said K.T., who snapped a picture of a bullet-riddled wall near the shooting scene. "It's one of those things you see on TV but never expect you'll go through."

Colin Ready, another employee, said he heard a couple of booms and thought the sounds were from construction. Then, there were several more booms and people were screaming and scattering, he said.

The manager of the store where Ready works closed the front gate, he said. When a police officer, not in uniform, came to the gate later to say the mall had been secured, Ready said workers didn't believe he was a cop.

"I have never experienced anything like this in my life," she said. "I was standing there talking to a customer and I started hearing all this banging coming from the food court... and people started running and they said somebody's down there and they got a gun."

Near tears, she added: "This country needs a lot of help. When somebody is that angry to go to a mall on a Saturday morning and shoot people. We're in a lot of trouble... To push people to those limits where things like that happen makes no sense."

Images on Twitter reportedly from the scene showed mall employees and customers hiding in a stock room. The mall was on lockdown for some time.

In the moments after the shooting, the fire department, via Twitter, advised people to avoid the area.

"I'm terrified,"said Lauryn Stapleton, a mall employee who barricaded herself in a room for an hour and half after the shooting."It's just something I never want to experience again. I'll go back to work but it will be hard."

Several university shootings earlier this week

Federal law enforcement officials said Saturday night that the Bureau of Alcohol, Tobacco, Firearms and Explosives is tracing the shotgun found with the shooter, who has not been publicly identified.

The Mall in Columbia released a statement hours after the shooter announcing it would remain closed for the remainder of Saturday. Police said later that the mall would be shuttered Sunday as well, while K-9 units continue canvassing the mall "through the night" to make sure all was safe.

"We are deeply saddened by the violence that has occurred this morning within our store in Maryland at The Mall in Columbia," Zumiez CEO Rick Brooks said. "We're making arrangements for counselling to be made available to Zumiez employees in the area."

The mall shooting was the latest instance this week of gun violence or threats of it in ordinary places across the country.

A student was shot dead Friday afternoon at South Carolina State University, prompting a manhunt for several suspects that extended beyond the school's Orangeburg campus.

On Wednesday, the University of Oklahoma in Norman briefly shut down after a report of a possible shooting that apparently turned out to be a false alarm, the university's president said.